Separate helper function for ec_pubkey_tweak_add
Separate helper function for ec_pubkey_tweak_add

This is in preparation for allowing code reuse by xonly tweak add functions

afk11/secp256k1,evoskuil/secp256k1,afk11/secp256k1,ElementsProject/secp256k1-zkp,bitcoin/secp256k1,evoskuil/secp256k1,bitcoin/secp256k1,ElementsProject/secp256k1-zkp,bitcoin/secp256k1,bitcoin-core/secp256k1,afk11/secp256k1,peterdettman/secp256k1,afk11/secp256k1,bitcoin/secp256k1,evoskuil/secp256k1,bitcoin/secp256k1,peterdettman/secp256k1,evoskuil/secp256k1,bitcoin-core/secp256k1,evoskuil/secp256k1,afk11/secp256k1